Placing concrete for a massive concrete building work, e.g., a dam, footing for a beam of a large bridge, or foundation of a high-rise building, LNG tank, or nuclear power plant, causes the heat of cement hydration to accumulate the concrete being hardened. Thus the temperature inside the mass rises as the concrete hardenes, whereas that of the surface portion remains nearly as low as the ambient temperature. The temperature difference between the two induces a difference in thermal expansion coefficient and hence cracking.
To overcome this difficulty, cements of lower calorific values and processes for producing them have been studied. Some approaches thus far made include changing the proportion of a mineral contained in portland cement (e.g., the choice of dicalcium silicate that slightly generates the heat of hydration as a chief component), altering the particle size distribution in cement, and reducing the water/cement ratio when kneading mortar or concrete on the spot (the textbook for the 249th Concrete Institute Class sponsored by the Cement Association of Japan, pp. 35-43 (1990)).
As compositions of the CaO-SiO.sub.2 -Al.sub.2 O.sub.3 system to which the present invention is related, slags and composite cements mixed slag and cement are already known.
A typical piece of the literature pertaining to the subject is the Proceedings of General Meeting/Technical Sessions, C. A. J., vol. 11, pp. 125-133 (1957). The literature states that a cement manufacturer made as samples various composite cements (portland blast-furnace slag cements) of commercially available slag and portland cement in mixing ratios (by weight) of 30-70:70-30, produced mortars using the above composite cements, formed hardened samples from the mortars, measured their compressive strength (.sigma.n)/heat of hydration (Hn) ratios at the age of 90 days, and obtained values ranging from 5.0 to 6.8. The reference cited concludes that those composite cements were not necessarily of low heat type and that if they were to be comparable in heat of hydration to a moderate heat portland cement it would be appropriate to adjust the proportion of the slag to the composite cement in the range of 50 to 60%.
The slag introduced by the reference is of a common type, with a composition of 38.7-41.9% CaO, 31.8-34.3% SiO.sub.2 (CaO/SiO.sub.2 (molar ratio)=1.21-1.41), 14.4-19.2% Al.sub.2 O.sub.3, and the remaining several percent MgO and other impurities.
Another piece of the literature to be cited here is Proceedings of General Meeting/Technical Sessions, C. A. J., vol. 6, pp. 49-56 (1952). According to the publication, varied compositions of the CaO-SiO.sub.2 -Al.sub.2 O.sub.3 system are synthesized by melting and quenching raw materials, and slag powders are formed by grinding those compositions to specific surface areas of about 3100 cm.sup.2 /g. Next, those slag powders and portland cement (addition) are mixed in a ratio by weight of 0.8:0.2 to prepare composite cements. Composites of mortar were made using the composite cements, each in proportions of sand/composite cement=1 and water/composite cement=0.45. Each composite of mortar was molded to form cylindrical pieces of hardened mortar 1 cm in diameter and 2 cm high. The publication shows the compressive strength values of those pieces were determined at the ages of 1 week and 4 weeks. It states, as a conclusion, that an optimum slag composition that exhibits a relatively high compressive strength is 47-52% CaO, 33-37% SiO.sub.2 (CaO/SiO.sub.2 (molar ratio)=1.36-1.69), and 14-18% Al.sub.2 O.sub.3. As for the heat of hydration, the literature is silent.
Moderate heat portland cement, one of typical compositions for low heat cement, gives low compressive strength values of 100-200 kg/cm.sup.2 and 300-500 kg/cm.sup.2 at the ages of 1 week and 13 weeks, respectively, while the hydration heat values are considerably high at 50-70 cal/g and 80-95 cal/g. The compressive strength/heat of hydration ratio at the age of 13 weeks ranges from 3.1 to 6.3.
Thus, when the cement is used for the construction works referred to above, it is customary, after the concrete composite has been placed and hardened, to cool it by water spraying or, alternatively, to place the composite partly, allow it to stand for a period long enough to dissipate the heat out of the hardened concrete mass and lower the internal temperature, and then place a fresh concrete composite to an adjacent space integrally with the pre-placed mass. Either practice has a disadvantage, calling for a watering-cooling step or considerable prolongation of the work period due to the natural cooling.
Among other known cements are composite cements of the portland cement-blast-furnace powder-flyash system and cements based chiefly of dicalcium silicate. Seriously low compressive strength practically bars their use for massive concrete structures; they find use in but small quantities for applications under special conditions.
As stated above, the cements of the prior art have failed to satisfy the both requirements of compressive strength and heat of hydration at extended ages for use in massive concrete construction works. There has been demand, therefore, for a novel cement that will replace the existing ones.